1. Field of the Invention
The present invention relates to the synthesis, purification, processing and evaluation of materials with very low permittivity to electric fields, and to their precursors. Such materials are critical to contemporary technology for improving performance in microelectronics (faster and smaller computers), communications (antenna coatings and radomes with greater ranges for weaker signals) and other applications.
2. Description of the Related Art
The importance of low dielectric materials to microelectronics and communications was recognized many years ago, but in recent years, with emphasis in higher speed, smaller size, higher frequencies and lower operational power requirements, it has assumed a more critical role in advancing these technologies. See A. von Hippel, Dielectric Materials and Application (MIT Tech. Press, 1954); C. P. Smyth, Dielectric Behavior and Structure (1955); S. J. Mumby, "An Overview of Laminate Materials with Enhanced Dielectric Properties", J. Electronic Materials, 18, 241, (1989); and R. R. Tummala and E. J. Rymaszewski, Microelectronics Packaging Handbook, (Van Nostrand Rheinhold, 1989). In microelectronics, low dielectric materials separate conducting lines and directly influence the speed of the device operation and the heat build up within it. In communications, low dielectric materials in the form of electromagnetically transparent coatings and shells (radomes) protect sensitive antennas from weather elements, corrosion and other airborne debris, and their reflection and absorption characteristics influence the sensitivity of reception and transmission. In other applications, low dielectric materials efficiently transmit electromagnetic radiation to an absorbing filler with a minimum of reflection.
The dielectric constant and dissipation factor (loss tangent or tan.delta.) are the parameters that describe the utility of low dielectric materials to serve in the above applications. Physically, the dielectric constant measures a material's ability to resist a flow of charge in a specified electric field relative to that of a vacuum. When the electric field oscillates, the current resulting from the flow of charge in the material has a charging component and a loss component. The dissipation factor is the ratio of the loss current to the charging current, and is a measure of the stored electromagnetic energy that is converted to a leakage current and ultimately dissipated as heat. These two parameters factor differently into various applications and will vary with frequency, temperature, and moisture content. In all cases, however, it is desirable to have them as low as possible. As used herein, a low dielectric material is one with a dielectric constant .ltoreq. about 2.7 (a low dielectric constant) and loss tangent .ltoreq. about 0.002 (a low loss tangent).
As a component of a microelectronic device, a low dielectric (or low permittivity) materials serves as an interconnect material which separates conducting lines within a multichip module or between modules in a printed circuit board. The influence of the interconnect material on the performance of the electronic device is determined by the effects of the dielectric constant and loss tangent on the signal propagation delay, crosstalk and dielectric losses. When an electrical signal propagates down a given conducting line, it generates an electromagnetic field which permeates into the surrounding interconnect material. The propagation time of the signal is determined by length of the conducting line and the velocity of the signal. A dielectric interaction between the electric field from the signal and the surrounding dielectric medium detracts from the signal's energy and slows its velocity. The size of this effect is determined by a reciprocal square root dependence on the interconnect dielectric constant, thus lowering the dielectric constant will increase the signal transmission speed. Also, the distance to which the electric field permeates into the interconnect material varies with the dielectric constant. If conducting lines are positioned too close together, crosstalk will occur: the electric field generated by a signal in one line can induce a signal in another line. Reducing the dielectric constant of the interconnect reduces the required distance between conducting lines, which translates into a smaller, faster device. The dielectric loss or signal attenuation (decibel/unit length) of a conducting line is directly proportional to the dissipation factor and to the square root of the dielectric constant. This third effect also relates to operational heat build up.
In communications and radar applications, low dielectric materials serve as antenna coatings and radome skin components. Again the dielectric constant and loss tangent are critical material parameters. Large differences between dielectric constants at the air-material interface cause reflections resulting in signal loss and signal distortion. Transmission of the electromagnetic wave through the material is accompanied by a power loss due to absorption of the signal. This absorption results in heating of the material which may become severe in high power transmissions. The minimization both of the dielectric constant to reduce reflections, and of the loss tangent to reduce absorption are critical to this application. Moisture absorption and impact resistance are additional critical properties. Small quantities of adsorbed moisture cause very large increases in the dielectric constant and loss tangent. On aircraft flying at sonic and supersonic speeds, the impacts of rain, ice and small debris can cause severe physical damage.
For other applications, the low dielectric material may be a coating and/or a binder for an electromagnetic absorbing material. As such, the critical requirement is minimization of electromagnetic reflection at the air material interface which translates to a minimization of the dielectric constant.
The features of chemical composition and structure that minimize the dielectric constant and loss tangent in a material are those associated with minimal polarization in an electrical field. This relates to permanent and induced dipoles in the material and the symmetry in which they occur.
Permanent dipoles result from bonds between atoms of unequal electronegativity. Permanent dipoles tend to orient in response to an electric field. Where dipole movement is possible (as in liquids and low glass transition materials), sizeable dielectric constants and loss tangents are observed with a strong frequency dependence correlating with the time scale of molecular movement.
Induced dipoles are created by the electric field and are strongest in materials with weakly held or delocalized electrons. This includes large atoms such as those below the first row in the periodic table and multiple bond systems such as aromatic structures. Strongly electronegative atoms like fluorine are particularly resistant to polarization. Saturated fluorocarbon and hydrocarbon polymers have very low dielectric constants (i.e. 2.0 to 2.3) while polymers with polar functional groups, particularly those with a strong affinity for moisture, have significantly higher dielectric constants (i.e. greater than 3.0). Also, solid-state packing density and symmetry factor into the dielectric constant. Increasing free volume as in the case of polyisobutylene (2.2) versus polyethylene (2.3) will reduce the dielectric constant, and a symmetry where a dipole associated with a bond is canceled by opposing orientation of similar dipoles as in the case of the cyanurate structure in the cyanate resin.
Depending on the application, other properties are also desired. These include facile processing, thermal stability, moisture resistance, good mechanical properties, and adhesion.
Most types of polymer processing, particularly for thin film depositions or castings, require that the polymer is at least temporarily in a fluid state. Consequently, a broad processing window (a function of time and temperature) where the polymer is in a fluid state, is desired. In microelectronics, a spin coating deposition is commonly used, where the polymer is dissolved in a solvent. This solution, or a prepolymer made from it, is applied to the surface of a microelectronic substrate, and then spun from this substrate at high rpm, leaving a residual thin film of uniform thickness. For thermoplastic polymers, the typical final step is evaporation of any residual solvent. For thermoset polymers, the typical final step is curing the low molecular weight prepolymer initial to a macromolecular network. A prepolymer is a partially reacted thermoset system that has been cured to below the gel point and is still melt and solution processable. Preferably, prepolymers are cured to just short of the gel point (e.g., to within about 10% of the gel point). The gel point is the point where "a system loses fluidity during polymerization . . . and passes over from a viscous to a gel-like state". A. Tager Physical Chemistry of Polymers p. 359 (Mir Publishers, Moscow, 1979) (2d ed.). Thermosets have the advantage of resistances to solvents and temperature, but the disadvantage of entailing more complex processing. This processing requires that stoichiometry not be upset by fractionation during spin casting when more than one monomer is involved.
Different applications will require different degrees of thermostability. In microelectronics applications, stability throughout a soldering operation (10 to 20 minute duration of 300 to 425.degree. C.) is desired. In radome applications, sustained thermal stability at an aircraft leading edge (about 150-200.degree. C.) is desired.
Moisture resistance is critical as a few percent absorption will seriously deteriorate the electric properties. Moisture will also advance corrosion at metal interfaces.
The important mechanical properties are mostly an impact or brittleness resistance for radomes to rain/ice and for circuit boards to drilling operations. At interfaces, adhesion to substrates or reinforcements must be sufficient to withstand subsequent processing and operational stresses. Also the interface must be stable to metal ion migration in the microelectronics operation.